1. Field of the Invention
The present invention relates to a wireless telegraph usable for a plurality of different communication systems in a mobile digital communication portable terminal.
2. Description of the Related Art
Hereunder, description will be made for both configuration and operation of a wireless telegraph used in a related art portable telephone. The wireless telegraph is divided into a transmitter and a receiver in the description.
At first, the transmitter of the related art portable telephone will be described.
In the transmitter, signals I and Q are entered to an input terminal of a signal modulator that can modulate a plurality of different signals. Passing the signal modulator, the frequencies of both the signals I and Q are changed in a multiplying circuit of a mixer according to a signal output from the first local oscillator.
After this, the signals I and Q pass a transmission band-pass filter, so that only a desired frequency is amplified in a power amplifier. The frequency-amplified signal then passes an antenna switch, so that only a desired signal is output from an antenna terminal.
Next, the receiver of the related art portable telephone will be described.
At first, the receiver receives a desired signal via the antenna terminal. Then, the received signal passes the antenna switch and a receiving band-pass filter, so that the received signal is amplified by a low noise power amplifier.
After this, the frequency of the received signal is changed by a multiplying circuit of a receiving mixer. Then, the received signal passes the second receiving band-pass filter that passes lower frequencies than the receiving band-pass filter. The frequency of the received signal is then changed using a frequency from the second local oscillator. After this, the received signal passes the third receiving band-pass filter, so that the received signals I and Q are demodulated with two types of phase signals.
On the other hand, there have been many types of portable telephone service communication systems. For example, the GSM system is used widely in Europe and the PDC and PHS systems are used widely in Japan, etc.
When using each of those communication systems, therefore, the user must prepare portable telephones as many as the number of communication systems to be used. There has been no compact and light-weight portable telephone that can correspond to such a plurality of communication systems.
If the configuration of the related art portable telephone is used as is for services of a plurality of communication systems, the number of parts in such a portable telephone will become double those of a portable telephone for a single communication system even in a rough estimation. And, the size of the portable telephone itself will become about double and accordingly, the telephone will become more heavy.
Furthermore, in order to meet the above prerequisites, it is needed to develop and mount devices such as a wide-ranged oscillator, a mixer, each band-pass filter, an amplifier, etc. that are usable commonly for the frequency configuration of each of those communication systems. In addition, each of those devices must be reduced much more in both size and weight. Otherwise, they are not usable for such a compact and light-weight portable telephone. The related art portable telephone has been confronted with such problems.